The present invention relates to a sintered hard alloy with superior corrosion resistance and wear resistance and also having high strength, hardness, fracture toughness, and corrosion resistance in a wide temperature range from room temperature to high temperature, which comprises a hard phase consisting mainly of the Mo.sub.2 NiB.sub.2 type complex boride and a binding phase of Ni-base metallic matrix which binds the hard phase.
The demand of wear resistant materials grows intensively year after year and materials having not only wear resistance but also corrosion resistance, heat resistance, fracture toughness, and high strength and hardness at high temperature as well as at room temperature are desired. Conventionally, WC-base cemented carbide or Ti (CN)-base cermet has been well known for wear resistance applications. However, they have shortcomings for usage because of insufficient corrosion resistance, strength, and hardness in a corrosive environment or a high temperature region. Focusing on superior characteristics of borides such as high hardness, high melting point, and electric conductivity and so on, a sintered hard alloy which makes use of metal complex borides such as Mo.sub.2 FeB.sub.2 and MO.sub.2 NiB.sub.2 and so on has been proposed as a substitutional candidate for conventional hard materials in recent years.
In these materials, a Mo.sub.2 FeB.sub.2 type hard alloy comprising a binding phase of a Fe-base matrix (Japanese Patent Publication Sho 60-57499) has insufficient corrosion resistance. On the other hand, a Mo.sub.2 NiB.sub.2 type hard alloy comprising a binding phase of a Ni-base matrix (for examples, Japanese Patent Publications Hei 3-38328, Hei 5-5889, and Hei 7-68600) which was invented for the purpose of improvement of corrosion resistance of the Mo.sub.2 FeB.sub.2 type hard alloy has superior corrosion resistance and heat resistance but has insufficient strength at room temperature.
Moreover, a Mo.sub.2 NiB.sub.2 type hard alloy which is disclosed in laid-open Japanese Patent Publication Hei 5-214479 has accomplished high strength while maintaining superior corrosion resistance and heat resistance by controlling the crystal structure of the boride constituting a hard phase as the tetragonal structure. However, the wear resistance of this hard alloy mainly depends on hardness, that is to say, the amount of the hard phase comprising the boride. Therefore, increasing the amount of the hard phase for the purpose of improving wear resistance leads to the tendency of decreasing strength and fracture toughness.
Consequently, materials with all superior characteristics such as high wear resistance, corrosion resistance, and heat resistance and high strength and toughness have not been obtained yet.
The objective of the present invention is to develop an alloy having the characteristics of Mo.sub.2 NiB.sub.2 type hard alloy as mentioned above, especially, high hardness, strength, and fracture toughness and the challenge of the present invention is to provide a sintered hard alloy having not only wear resistance, corrosion resistance, and heat resistance but also sufficient strength and toughness in a wide temperature range from room temperature to high temperature, high strength, high toughness and high corrosion resistance.